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POWER SUPPLY DESIGN PDF

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For this step by step example we'll design an Start: Start your design by deciding what your power supply requirements . and/or print a pdf version of the . Transformerless AC power supply theory is not generally taught at the university the design process, yet these sources emphasize the math without explaining. As most electrical system design engineers have experienced, power supply design is often left until the last minute. With a deadline looming and the boss.


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POWER SUPPLY DESIGN BASICS by P. ANTONIAZZI. In mains-supplied electronic systems the AC input voltage must be converted into a DC voltage with the. Linear Power Supply Design Examples Elementary Discrete Linear Regulator Designs Basic 3-Terminal Regulator Designs Power supplies often seem like an imposition - most circuits need one, and they The basic procedure for power supply design is to first build an unregulated.

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Protection diodes are. One last caution is the little-known breakdown. If the output voltage exceeds V , more than one.

The MC can also be used to. BUX85 R sc. Several design restrictions must be.

The TIP50 provides the bias supply for the controller, which must withstand the. The controller is. T o readjust the output voltage, one changes the value of the two series resistors.

Floating linear regulators are particularly suited for high-output voltage regu-. This regulator can be seen in Figure 2—9. Pulsewidth Modulated Switching. Although pulsewidth modulated PWM switching power supplies have been. Switching power supplies offer many advantages. T oday, there are two ways to approach the design of switching power sup-. However, if any of the require-. This book is organized so that the massive process of designing a custom.

The intent is for the reader to read the section, choose the best design. The design order is the way that seasoned power engineers use to approach. The operation of switching power supplies can be relatively easy to understand. Unlike linear regulators which operate the power transistor in the linear mode,. In these states, the volt-ampere product across the. This EI product within the power device is the loss within all the power.

Once the input voltage is converted to an ac rec-. Additional output voltages can be derived by adding secondaries to the trans-. The controller, whose main purpose is to maintain a regulated output voltage,. That is, the functional blocks,. There are two major operational types of switching power supplies: Although their arrange-. Forward-mode regulators form a large family of switching power supply topolo-.

A simple form of.

Practical Switching Power Supply Design / Marty Brown.

This is called the buck. The output voltage is maintained by the controller by varying the duty cycle. The buck converter is also known as a step-down converter , since its output must. The operation of the buck regulator can be seen by breaking its operation. When the switch is turned on, the input. The inductor current ramps. I on I off. Figure 3—1 A basic forward-mode converter buck converter shown.

The energy stored within the inductor during this period is. When the power switch is turned off, the input voltage to the inductor wants. The current waveform, this time, is a negative linear ramp whose slope is. When the power switch once again turns on, the diode snaps off and. The dc output load current value falls between the peak and the minimum. In typical applications, the peak inductor current is about The advantages of forward-mode converters are: Forward-mode converters can. Figure 3—2 The voltage and current waveforms for a forward-mode converter buck converter.

The transformer offers some dis-. The second family of converters are the boost-mode converters. The most ele-. As one can notice, the boost-mode converter has the same parts as the.

This new arrangement. This time, when the power switch is turned on, a. The diode is reverse-biased during this period. Figure 3—4 Waveforms for a discontinuous-mode boost converter.

Figure 3—3 A basic boost-mode converter boost converter shown. When the. The inductor voltage is then clamped at the value. This is seen in the inductor. When the core does not com-. This is. The energy stored within the inductor of a discontinuous-mode boost con-. This means that the energy. L off pk on. Switch V oltage. Figure 3—5 Waveforms for a continuous-mode boost converter. The boost converter shown in Figure 3—3 can only be used as a step-up con-. That is, the output voltage must be higher than the highest value of input.

If the inductor is replaced by a transformer, as seen in Figure. The transformer also provides a. The outputs also become indepen-. Due to the higher peak currents within boost-mode converters, they can only. They have the least parts of all. PWM switching power supplies lend themselves quite nicely to an organized. They are more complicated and therefore can be par-. The switching. Some initial decisions must be made at the beginning,.

Once the topology is selected, the design path is determined and the design. By proceeding through the block diagram in Figure 3—7, in the. Each functional block in Figure 3—7 will have a choice of typical. The designer determines from his or her. Then, by executing the design equations and using the parameters supplied by.

A major decision that must be considered at the beginning of a switching power. The term topology refers to the. This arrangement has a large bearing on which environment the supply. This is the point in the design process where the major cost versus per-. Each topology has its relative merit. One topol-. More than one. A summary of the relative merits of the.

The major factors that determine the optimum choice of topology are:. Is transformer isolation needed from input to output? How much input voltage appears across the primary winding of the trans-. What is the peak current through the power switches? What is the maximum operating voltage across the power switches? The nontransformer isolated topologies are used for board-level converters. These are in distributed power systems where an intermediate bus voltage is.

I still. The added cost. Figure 3—7 The functional block diagram for a PWM switching power supply. T ransformer. The amount of voltage appearing across the primary of the transformer is. Switching power supplies are constant power circuits. That is, the lower the. Above 20 A,. By using another topology, the peak current can. The higher the maximum voltage the power switches experience, the greater. V oltage. For transformer isolated topologies, the industry has.

But because its peak currents are much higher than the forward-mode con-. Between an output power of and W the half-bridge see Figure. The parts cost more but they are still reasonable. The half-bridge converter only places one-half of the input voltage across the. It therefore is. Above W and into many kilowatts, the full-bridge. This requires four power switches, two of. The push-pull see Figure. This will cause the. Pulse-to-pulse current-.

By reviewing T able 3—1 and Figure 3—8, you can develop a good idea as to. The appropriate topology then can be chosen. Figures 3—9 through 3—16 show the basic PWM switching power supply. Output Power W atts. DC Input V oltage Volts. Figure 3—8 Where various topologies are used. Figure 3—9 The buck step-down converter. Figure 3—10 The boost step-up converter. Figure 3—11 The buck-boost converter. C in Control. SW C out. Control SW D. V in C in Control. C out V out. Figure 3—13 The one-transistor forward converter.

I SW1 I pk. Figure 3—14 The push-pull converter. I min -. V sat SW2. Figure 3—15 The half-bridge converter. V sat SW2—3. Figure 3—16 The full-bridge converter. This preliminary step in the design phase, predetermines some of the major. This allows the designer. It also results in knowing some of the particularly important parame-.

Average input currents. Solve for the highest input average input current that occurs at the lowest. This value allows the designer to determine the size of. The input peak current. This is completely determined by the topology previously chosen. For the forward-mode supplies, it is just a curios-. Each topology has predictable voltage and current stresses for the power. Selecting the power devices at this stage in the design cycle can save. T able 3—2. Estimating component losses optional.

The relative losses among the various sections within a PWM switching power. These loss proportions are. V out 1. V out n. When the topology has multiple power switches, then multiply the P loss ckt. That is, do some of the semiconductors need.

Also, a guess at how much heatsinking is required. This completes the black box estimates portion of the design program. As one. The design of the magnetic elements forms the backbone of a good switching. Their proper electrical and physical design have a large. Since switching power supplies are a specialized narrow application of magnetic. This yields the quickest working design without having to under-. For more detail in. Part of the confusion in designing the magnetic elements is the inexactness.

At best, the results of calculations should be considered. These trade-offs will be discussed at the appropriate points in the.

Select the core material appropriate for the application and for the fre-. Select the desirable core style that will meet the needs of the application. Determine the size of the core needed to provide the required output. Determine whether an airgap is needed and calculate the number of turns. Then determine whether the accuracy of the. Wind the magnetic component using the described physical winding tech-.

During the prototype stage, verify its operation with respect to the level.

If the above procedure is executed, the initial design of any magnetic com-. Please refer to the end of Appendix. Each manufacturer uses different core sizing procedures. Some use graphs,. The following two procedures are generalized approaches for. The following methods take two forms. The results of both should be considered estimates, so. This will eliminate the need to go back later if the windings. Parameter Affected:.

Output power A c core cross sectional area. Input voltage A w window area. Number of windings A w window area. Other considerations enter into the operation of the magnetic element, such. Core Sizing Method 1. Order those samples refer to T able. This method begins with the assumption of a one-winding transformer. P out is the total output power of the supply. For the MKS pure metric system the following equation is used:. P out is the total output power of the supply W. These scaling factors are found in T able 3—5.

Combine the individual scaling factors as follows:. WA Pd. In the U. The relationship to convert between the two numbering system. The core with. This phase of the transformer design is a gross. There are two main functions the forward-mode transformer performs: The forward-mode transformer stores no energy other than a small amount. There are only two major. Do the resulting windings provide an accurate enough output voltage to. Other considerations are important during the physical winding process and.

For this, the parameters from the core data sheet of the particular core. The equation for. T able 3—5 T ransformer Growth Scaling Factors. Consideration Scaling Factor. Flyback transformer 1. One secondary 1. T wo or more secondaries 1.

Isolated secondaries 1. UL or CSA approval 1. Faraday shield 1. V in nom is the typical operating input voltage V. Some core companies use still a third metric system, milliT eslas mT , and. These companies tend to be Japanese. This number of turns now serves as the reference winding upon which all the.

Next, one determines the number of turns of the highest power output sec-. The equation to determine the number of turns for this winding is. DC max is the maximum expected duty cycle 0. V in min is the minimum expected input voltage. This equation can be solved for the needed secondary turns at the lowest. At any input voltage lower than that, the regulator.

The next step is to determine the number of turns for the other secondaries. The starting point is. The result of this calculation will always yield a noninteger number, but many. Therefore, one must round. This results in an error in the eventual. One must now check to see if these. If the error in any of the output voltages is too great,. If the result is still unacceptable, go back to the.

Remember that. This iterative process should continue. The designer will have to accept some error in the output voltages as. The next step is to decide how the secondary windings should be arranged. That is, whether it is desired to have isolated secondaries, center-tapped or non-. Also, autotransfomer windings have more. Instead of both windings conducting current simultaneously, energy is. Accordingly, the classic trans-. Now the voltage, time, and energy become the main.

T o start the design, the peak current should have been estimated during the. The core style and material should. This time an air gap is neces-. One begins by realizing that the primary winding behaves like an elementary. This inductance value should be considered the maximum value since any. The energy entering the core during each on time of the power switch is. Figure 3—18 Forward-mode secondary winding arrangements: T o verify that the maximum continuous output power capability of the trans-.

This is an estimated air-gap length. The designer should use the closest value. The core manufacturer provides a parameter called A L for each airgap length. This parameter is the inductance of the core when 1, turns is placed on it.

T o determine the number of turns required to achieve the desired value. This value of N pri represents the maximum value of primary inductance that can. Now determine the turns of the highest power secondary winding by using. The result of Equation 3. This calculation will always result in a non-. Many cores do not support fractional turns, so round. T o determine the turns for any additional windings, use the procedure given.

Once again, if the. Start by remov-. Eventually, some. The arrangement of the secondary should now be considered. The designer. When the desired secondary winding. This is done in the same fashion as seen in Equa-. If they are not properly designed physically, excessive voltage spikes could be. Figure 3—19 Flyback transformer secondary arrangements: Its purpose is to store energy for the load during the periods. Its electrical function is to integrate.

Its design is somewhat simple. First, the core should be chosen. Usually, a. This is because this material is. The following method will demonstrate how. V out is the output voltage. T off est is the estimated on time of power switches at the highest input. I out min is the lightest expected load current for that output. This value represents the minimum value for the inductance, below which the. For mopermalloy toroids, the method for estimating the needed core size is. Refer to Figure 3—20 and locate this value on the x-axis and move vertically until.

Then move horizontally and read the part number. Equation 3. The percent of occupied window area is determined. A window is the available wire area of the toroid window area in 2 or. N is the number of turns. If this value is greater than 40 to 50 percent, then too much of the window is.

The solution is to go to the next larger core size or to drop. The latter will increase the temperature of the induc-. Lastly, if the supply operates at a high frequency and the current through the.

Litz wire has an overall. Within multiple-output forward converters, it is possible to easily combine the. This offers several advantages; it saves space, vastly. First the core style and material should be selected. This is done in an iden-. Either a mopermalloy MPP. For the MPP toroid core, determine the size of the core needed,.

The necessary core winding area. The number of turns for both windings is determined by calculating the.

This is done using Equations 3. The other winding will have the. This guarantees. A twist pitch. Since the. If the windings are mistakenly. I highly recommend against this temptation.

If the windings are not exact to. The common choice of core is the MPP toroid core. These cores have distributed air-gaps within the core material and come in. The rule of thumb is that the higher the dc currents. The core.

A permeability of 60 or below is. This can be done by. Litz wire is not needed since the amount of ac passing through the.

Next refer to the normal magnetization curves and select a value of H mag-. From the. Choosing a permeability. Next comes an iterative process. A winding factor of 50 percent or less is good. Calculate the wire area by multiplying the chosen wire cross-.

For the initial choice of core, calculate the actual number of turns needed by. If it is larger go to the next larger core, and if it is less than 30 percent. Then recalculate the new turns for that core. The purpose of gate or base drive transformers is to provide isolation between. Their design is relatively easy,. There are several important factors to consider during the design of the gate.

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